A liquid crystal display device such as a tablet PC, a smartphone or the like employs a light guide plate (light guide film) of a edge-lit type as a back lighting device. An edge-lit-type light guide plate emits light incident on one edge (light incident surface) of the transparent square plate from its surface (light emitting surface). A back lighting device has a light source arranged on at least one edge of the light guide plate. The surface opposing the light emitting surface of the light guide plate (light-reflecting surface) is provided with an element for changing the advancing angle of the light transmitting inside the light guide plate (hereinafter, referred to as a “polarizing element”). Light incident on the light incident surface of the light guide plate is reflected by the light-reflecting surface, by which the advancing direction of the light is changed and the light propagates within the light guide plate and is emitted from the light emitting surface. In general, the light guide plate is provided with a polarizing element whose density distribution and shape are determined such that the brightness of the light is uniform over the entire light emitting surface.
A method for manufacturing such a light guide plate is generally an injection molding method like the method disclosed in Japanese Unexamined Patent Application Publication No. Heisei 9-222514. A light guide plate manufactured by an injection molding method is restricted in thinning the plate thickness due to limitations in fluidity, moldability and the like, and thus the thinner side of the plate thickness is 0.3-0.4 mm at the thinnest. In addition, since the size of the light guide plate is increased along with the increase in the size of the liquid crystal screens, a larger injection molding machine is necessary, which is associated with problems such as an increase in the manufacturing cost, requirements of excessive pressure, relatively long pressurizing time and cooling time to ensure the transfer accuracy, and the like.
As a method for solving such problems, a method for manufacturing a non-uniform-thickness film with an extruder has been proposed, in which at least two non-uniform-thickness films are extruded to be integrally molded such that they are symmetrically arranged side by side, like the method disclosed in Japanese Unexamined Patent Application Publication No. 2004-82359.